In recent years, with the spread of fuel injection devices called injectors, the control of fuel injection timing and fuel injection volume, in other words, air-fuel ratio becomes easier, and this has enabled to promote increase in output, reduction in fuel consumption and purification of exhaust emissions. Among these, in particular, as to the timing of injecting fuel, it is a common practice to detect, strictly speaking, the state of an inlet valve, or, generally speaking, the phasic state of a camshaft so as to inject fuel to the state so detected. However, a so-called cam sensor for detecting the phasic state of the camshaft is expensive, and in particular, with motorcycles, there are many cases where the cam sensor cannot be adopted due to a problem of the enlargement of a cylinder head. Due to this, for example, JP-A-10-227252 proposes an engine control system for detecting the phasic state of a crankshaft and an intake air pressure so as to then detect the stroke state of a cylinder. Consequently, since the stroke state can be detected without detecting the phase of the camshaft, it becomes possible to control the fuel injection timing to the stroke state so detected.
Incidentally, in order for fuel to be injected from the fuel injection device as described above, fuel in a fuel tank has to be pressurized with a pump so as to be supplied to the fuel injection system. As has been known, since the pressure of fuel pressurized by the pump fluctuates, a pressure regulator valve called a regulator is used to regulate an upper limit value of the pressure of the fuel so supplied. In the case of motorcycles, it is a common practice to provide the regulator in the close vicinity of the fuel injection device, with an atmosphere into which fuel is injected by the fuel injection device, for example, a pressure within an induction pipe being normally used as a back pressure, a predetermined regulator control pressure normally set by a spring is designed to be raised from there. Consequently, the pressure of injected fuel which is made up of a differential pressure between the pressure of fuel supplied to the fuel injection device and the atmosphere into which fuel is injected is always equal to the regulator control pressure of the regulator.
When the regulator is provided in the close vicinity of the fuel injection device like this, however, a return piping for returning a portion of fuel which is restricted by the regulator to the fuel tank is required for each fuel injection device. In addition, generally, inmost cases, regulators are manufactured by a manufacturer of pumps, and in the event that the pump and the regulator are designed to be disposed separately, pumps and regulators are delivered separately, and this increases the number of components and makes it impossible to attempt to attain a reduction in production costs that would otherwise be attained by getting them assembled together. To deal with this, it is conceivable to dispose the regulator on a pump side or to get the pump and the regulator assembled together. With this construction, not only can the necessity of the return piping for returning fuel be obviated but also the reduction in number of components and in production costs can be attained.
In the event that the regulator is disposed on the pump side as has been described above, since the back pressure of the regulator becomes the atmospheric pressure, the pressure of fuel supplied to the fuel injection device becomes substantially constant (a change in atmospheric pressure due to change in altitude changes the pressure of fuel). On the other hand, as is the case, in particular, with motorcycles, in case there is no surge tank along the length of the induction pipe, the pressure in the induction pipe into which fuel is injected, namely the pressure in the fuel injection atmosphere tends to fluctuate. Namely, the pressure of injected fuel constituted by a differential pressure between the pressure of fuel supplied to the fuel injection device and the pressure of atmosphere into which fuel is injected becomes unstable. In the event that the pressure of fuel injected becomes unstable as has been described above, the flow rate per unit time of fuel injected from the fuel injection device becomes unstable, and, for example, it becomes impossible to perform a control over a fuel injection volume needed to attain a desired air-fuel ratio only by fuel injection time. Then, in order to correct and control the fuel injection volume based on the pressure of injected fuel as is described above, for example, JP-8-326581 describes an engine control system. In this engine control system, an injected fuel pressure is detected, and the injected fuel pressure so detected is then integrated a predetermined period of time to obtain an area. Then, the area so obtained is compared with a reference area so as to correct and control the fuel injection volume.
Here, since the injected fuel pressure is the differential pressure between the pressure of fuel supplied to the fuel injection device and the pressure of the atmosphere into which fuel is injected, for example, when the pressure of fuel supplied to the fuel injection device is a pressure restricted by the regulator, the pressure is a value resulting from addition of the atmospheric pressure to the regulator controlled pressure, and, strictly speaking, in case the atmospheric pressure is not accurate, this means that the pressure of fuel supplied to the fuel injection device is not accurate. Then, while the use of an atmospheric pressure sensor is considered, since atmospheric pressure sensors are expensive and elaborate, the application to motorcycles is limited. In addition, in, for example, an engine control system in which a fuel injection quantity is determined by detecting a volume flow rate of air which flows into the engine, the detection of atmospheric pressure is needed to correct air density, and an apparatus and method for detecting an atmospheric pressure have been desired which can replace the atmospheric pressure sensor.
The invention was developed to solve the problems, and an object thereof is to provide an apparatus and method for detecting an atmospheric pressure for a four-stroke engine which can accurately detect an atmospheric pressure without using an atmospheric pressure sensor and which can attempt to reduce the production costs as well as the number of components.